An optical pick-up is generally composed of an optical element such as a light-emitting element, a light-receiving element and a mirror, an optical frame for fixing these optical elements, an actuator part for moving an objective lens and an objective lens holder following the movement of the optical disk, and a base frame which maintains an actuator part and forms an optical pass to the optical frame.
The optical frame is required to have dimensional stability for maintaining a relative position of the optical elements at a constant relationship, and the base frame is required to have a mechanical strength so as to be free from resonance with the movement of the actuator. In many cases, articles molded by metal die casting such as aluminum are generally used for these parts.
Recently, engineering plastics are partially put into a practical use as a metal substitute for reducing the weight of the parts and lowering the cost of parts. However, in applying these plastics for thin portable type optical devices which require a higher strength of materials or for mounting these plastics on automobiles which are often encountered severe environmental conditions, more excellent mechanical strength and dimensional stability are required for the engineering plastics.
Known materials satisfying these requirements include polyphenylene sulfide (PPS) and liquid crystal polymers (LCP), but the liquid crystal polymers have problems in that anisotropy in elasticity and linear expansion coefficient is increased in a machine direction at the time of molding and a vertical direction thereto. Also, the polyphenylene sulfide resin composition described in JP-A-3-66756 (the term "JP-A" as used herein means an unexamined published Japanese patent application) does not show such an anisotropy observed in the liquid crystal polymers and is capable of producing a molded article having a high resonant frequency region due to its high elasticity. However, since the above-described known polyarylene sulfide resin composition employs a particulate filler having a high specific gravity of 3.5 or more, the particulate filler tends to be localized by precipitation due to its high specific gravity and, hence, a molded article in which the particulate fillers are dispersed stably is difficult to obtain. Such molded articles tend to cause deviation in the optical axis with changes in the environmental conditions and thus are not suitable to the use for optical pick-up parts which require high reliability.
Further, in the optical pick-up parts formed by using the polyphenylene sulfide resin composition, a large amount of fillers is generally used therein so as not to cause deviation in the optical axis with changes in the environmental conditions such as temperature and humidity and, as a result, the article molded from such a resin composition has a high brittleness and an insufficient tenacity. Thus, the molded articles have various problems such as generation of breakage in a mounting step due to insufficient thread cutting strength and generation of cracks in a metal press fitting.